1. Field of the Invention
This invention relates to methods and apparatus for fabricating compressed gas insulated transmission line (CGIT) systems for electric power applications.
2. Description of the Prior Art
In a typical CGIT system, one or more generally aluminum conductors are suspended within a metallic, generally aluminum sheath on dielectric spacers. The sheath is filled with a gas, typically sulphur hexafluoride (SF.sub.6) pressurized at about 50 psig. The spacers are typically cast epoxy posts or discs affixed to the inner wall of the aluminum sheath to maintain a fixed minimum separation between conductors and between the sheath and encased conductors. One form of a single conductor CGIT assembly is described in U.S. Pat. No. 3,864,507.
The major cost of a CGIT assembly lies in the fabrication of the aluminum sheath. The larger diameter aluminum sheaths, typically exceeding 21 inches in diameter, are difficult to construct according to conventional techniques. For such large diameter sheaths, spiral welding or rolling and welding have been suggested. Where still larger diameter sheaths are required, and particularly in multiple conductor systems, where for example three conductors are provided within a single sheath, assembly can be unwieldy and alignment difficult.
In order to facilitate construction of large diameter sheaths, it has been suggested that the sheath be fabricated in a plurality of modular elongated sections of semicircular cross section which may readily fit together.
The multiple section construction technique renders accurate cable alignment difficult. Improved methods are thus needed for extruding sheath sections wherein the alignment and mounting of cable spacers are simplified in order to optimize cable operation, save construction time and hence cost.